Contrast agents are microparticles detectable by imaging. The term “imaging” refers to detection using an imaging device, examples include but are not limited to, ultrasound or ultrasonic (US) imaging, magnetic resonance imaging (MRI), scintigraphy, single photon emission computed tomography (SPECT), positron emission tomography (PET), computed tomography (CT), X-ray imaging/fluoroscopy, fluorescence imaging, bioluminescence imaging, microscopy, optical methods, or multi-modal variants thereof.
Suitable contrast agents for (contrast enhanced) imaging depends on the nature of the imaging modality proposed, and vice versa. For example, gas-containing microparticles such as microbubbles may be used as contrast agents in US imaging; microparticles containing radionuclides (e.g., technetium-99m, thallium-201, iodine-123, iodine-131, gallium-67, indium-111, fluorine-18, carbon-11, nitrogen-13, oxygen-15, rubidium-82) may be used as contrast agents in scintigraphy, SPECT or PET; microparticles containing paramagnetic, superparamagnetic or ultrasuperparamagnetic materials (e.g., gadolinium (Gd), iron oxide, iron, platinum, manganese) may be used as contrast agents in MRI; microparticles containing radio-opaque materials (e.g., iodine, barium, metal) may be used in CT or X-ray imaging/fluoroscopy; microparticles containing fluorophores or fluorescent dye (e.g., fluorescein-5-isothiocyanate, rhodamine, 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate (DiI), 3,3′-dioctadecyloxacarbocyanine perchlorate (DiO)) may be used in fluorescence imaging/microscopy; microparticles containing enzyme substrates (e.g., that for luciferase) may be used in bioluminescence imaging. Contrast agents may be detectable by more than one imaging modality. For example, a microbubble with/without paramagnetic material may be detected by US imaging, MRI and microscopy.
The term “imaging signal” refers to the received signal in imaging, that is identified to represent that of a contrast agent or contrast agent plus another element (e.g., tissue or blood). The received signal includes, but is not limited to, the raw, radiofrequency or front data, data before/after coding or processing, image pixel data (or image), or the number/density/concentration of microparticles observed visually under microscopy.
The term “signal intensity” refers to the intensity or strength of the imaging signal, it may be used synonymously with similar terms such as (but not limited to), the signal amplitude, signal strength, signal power (eg signal voltage squared, signal audio loudness), signal decibel (dB), signal videointensity, signal videodensity (e.g., pixel intensity on an image in grey or other colour scale), or the number/density/concentration of microparticles observed visually under microscopy. Where appropriate, the image videodensity (e.g., pixel intensity) may be substantially linearised using a suitable function (e.g., decompression using an anti-log function).
Contrast agents which comprise molecular binding elements can be used in appropriate imaging modality/modalities for molecular imaging, for the detection of molecules of interest (target molecules). For example, US molecular imaging can be achieved using targeting microbubbles as contrast agents. Microbubbles are formed of a shell encapsulating a gas. The shell can be made of a lipid, protein or polymer. Microbubbles oscillate within an acoustic field producing signals appearing as bright spots on an US picture, thereby effecting US contrast enhancement. The microbubbles are sufficiently small to flow without obstruction through small blood vessels, rather like the way in which red blood cells flow. Targeting microbubbles have shells containing molecular binding elements, which bind to molecules of interest one wishes to detect. Thus, for example, when targeting microbubbles are introduced into the bloodstream, they circulate with the blood and attach and accumulate on and around the molecules of interest, detectable using US imaging. Non-targeting microbubbles can also be imaged using US molecular imaging.
The molecule of interest (target molecule, targeted molecule or molecular target) may be, but is not limited to, a molecule, protein, receptor, particle or cell (including that present on artificial/implanted materials, e.g., metal, polymer or drug on a coronary stent, prosthetic heart valve or closure device). The molecule of interest may be present on the surface of cells.
The molecule of interest may exist de-novo or may be introduced artificially into a subject or system.
The terms “contrast agent”, “microparticle”, “targeting microparticle”, “microbubble”, or “targeting microbubble” may be used synonymously.
“Targeted microbubble contrast enhanced ultrasonography” (targeted MCU) is a name given to such a technique whereby targeting microbubbles are introduced into a subject or system, and the microbubbles are imaged using an US device. Examples of a suitable device includes, but are not limited to, the Siemens Acuson Sequoia 512 ultrasound system (using, for example, its contrast pulse sequencing (CPS) imaging mode), Phillips HDI5000 ultrasound system (using, for example, pulse inversion imaging mode), Phillips Sonos 5500 ultrasound system (using, for example, power modulation imaging mode), or VisualSonics Vevo 770 (using for example linear imaging mode) or Vevo 2100 (using, for example, non-linear imaging mode).
A short period of time after microbubble administration, part of the microbubble population will have adhered to the molecules of interest, and are described as retained microbubbles, while others may remain free, described as free microbubbles. Retained microbubbles are microbubbles retained or accumulated in a tissue or system (for example a flow chamber system) due to adherence to the molecule(s) of interest. Retained microbubbles may also be retained in a tissue or system due to other mechanisms including, but not limited to, non-specific adherence or cellular-uptake. Free microbubbles are microbubbles that circulate freely in a tissue or system. Both retained and free microbubbles decrease in number over time owing to their elimination.
Targeted MCU has been used to determine the concentration of a molecule of interest (a target molecule), by measuring the retained microbubble signal intensity after a certain time following the introduction of the microbubbles such that the free microbubbles in the body or system have decreased through elimination to a relatively low level (for example when the signal caused by free microbubbles has become low, insignificant, minimal or undetectable). However, I have found this method of imaging signal analysis lacks sensitivity and has a low degree of quantification, as evidenced by it being poor at detecting low concentrations as well as small changes in the target molecule concentration. Furthermore, it is prone to inaccuracies, inconsistencies and wide variations. Alternative imaging signal analysis methods suffer from attenuation and/or saturation of microbubble ultrasound signals when microbubbles are at high or moderate concentrations, making signal analysis for the determination of the target molecule concentration very difficult or inaccurate. While these problems can be mitegated to some degree by reducing the number of microbubbles administered into the body or system so that signal attenuation and/or saturation either does not occur or is minimised, I have found that this reduces the number of target molecules that can be detected as well as the accuracy and degree of quantification of the target molecule concentration.
The foregoing identifies a major obstacle limiting the development of targeted MCU towards human application.